With the discovery of semiconductor materials and subsequent development of integrated circuitry there arose the need to grow large or defect-free crystals of elemental materials such as silicon (Si) and more recently compound materials such as gallium arsenide (GaAs). Crystalline structures are widely used in the manufacture of electrical circuitry, optical systems and other microminiature devices. Several techniques have been utilized for growing crystals, but none of them are believed capable of growing larger crystals of the compound systems now needed while maintaining low defect densities. A brief review of existing methods are hereinafter set forth.
The Czochralski method is one wherein a seed crystal is dipped into a container of molten material from which a crystal is to be grown, and the growing material is pulled vertically from the molten material. A difficulty with this method is that when applied to gallium arsenide (GaAs), the crystals typically have large defect densities on the order of 10.sup.3 -10.sup.4 /cm.sup.3. This limits its used in large scale integration circuitry despite the general advantages of gallium arsenide over silicon. Similarly, cadmium telluride (CdTe), which is used as a substrate in focal plane arrays, is subject to defects, such as dislocations, low angle grain boundaries, and twin crystal formation when the substrate crystal is grown by conventional techniques. Current methods used in the crystal growth process include several types of molten zone growth methods.
The Bridgman-Stockbarger method utilizes an elongated container of semiconductor material which is melted in a high temperature furnace, after which the container is lowered into a cooler, lower temperature furnace, which allows the material to slowly resolidify as a single crystal. The molten material from which the crystal is grown is completely enclosed during the process, and as a result, strains occur in the material which induce defects when the molten material solidifies.
In a method referred to as the horizontal Bridgman method, an open boat-type container is used to hold molten crystal material which is then slowly cooled to allow a crystal to form in the boat. This approach attempts to solve the problems referred to with respect to the Bridgman-Stockbarger method by holding the material in the open-boat container, allowing free expansion of the forming crystal when it solidifies. Convective flows are uncontrolled, however, resulting in impurities being transported from the walls of the container, causing undesirable doping of the grown crystal. Further, crystals grown by this method are D-shaped, which is undesirable for integrated circuit construction.
Finally, there is the float zone technique. It eliminates the container of the Bridgman-Stockbarger approach and uses a polycrystalline rod supported at each end which is passed through a region of high intensity heat. Float zone crystal growth does offer good possibilities for reducing growth defects since container walls are eliminated and since heat is added horizontally around the circumference. This creates radial thermal gradients in the melt which may be arranged to provide a convex growth interface. In fact, it has been possible to grow dislocation-free silicon (Si) by this process. There are, however, several limitations in extending float zone growth to other materials: (1) Si has an unusually large surface tension (10 times that of H.sub.2 O) which helps support large molten zones; (2) molten Si is a good electrical conductor which allows RF induction heating, which also provides additional Lorentz forces to help support the molten zone; and (3) even though it is possible to eliminate dislocations in float zone Si, there are severe growth rate fluctuations and thus compositional striations caused by the heating asymmetry inherent in the RF work coil and by uncontrolled convection driven by buoyant as well as surface tension forces. In fact, a recent space experiment indicated that surface tension-driven convection may be the dominant cause of such striations. The growth rate fluctuations are not particularly serious in an intrinsic elemental semiconductor such as Si but could produce serious inhomogeneities and defects in extrinsic, alloy type, or compound semiconductors. Another difficulty with conventional float zone growth of multicomponent systems is the control of stoichiometry since the more volatile component will tend to evaporate at the free surface. Liquid phase encapsulants cannot generally be used because of gravity drainage. Maintaining an over pressure of the volatile component may not be possible because of the temperature variations associated with the process since the maximum pressure of a particular component that can be obtained is the vapor pressure of the component in question at the temperature of the coldest region in the pressure vessel. Finally, the float zone process cannot be successfully applied to many materials on earth because of their low surface tension. This is especially true for poor conductors because Lorentz forces cannot be used to help support the molten zone.
It is the object of this invention to provide an improved apparatus for growing relatively large, defect-free crystals of compound materials.
Another object is to provide a method of growing such crystals.